World

Giant Planet Found Lurking Inside the Milky Way's Galactic Bulge

Astronomers have discovered a giant world orbiting a star using a quirk of Einstein’s general relativity. This “microlensing” event enabled astronomers to test a new survey technique to detect the alien world from more than 25,000 light-years away, deep inside the Milky Way’s galactic bulge.

Microlensing events occur when a star passes in front of another more distant star. As the nearer star passes in front, its gravitational field — which is (according to general relativity) bending the surrounding spacetime — deflects the light from the more distant star. Like the lens in a magnifying glass, the starlight is magnified and Earth-bound observatories are able to spot a transient brightening. Information about the “lens” (the foreground star) and any planets in tow can then be deduced.

The microlensing event “MOA-2011-BLG-322″ was detected during a 2011 observing season of a collaboration of observatories. Astronomers of the Microlensing Observations in Astrophysics (MOA — New Zealand/Japan), Optical Gravitational Lensing Experiment (OGLE — Poland) and Wise (Israel) all reported the event. Of the 218 microlensing events detected during that season, only 80 were confirmed by all three networks. Of those 80, three showed signs of a clear “planetary anomaly.”

A planetary anomaly is caused by a secondary mass (i.e. a planet) creating its own spacetime warping, adding detail to the microlensed starlight. Microlensing has therefore become a useful tool in the search for exoplanets orbiting distant stars. This technique contrasts greatly with the two leading techniques of exoplanet detection — the “transit” technique (the dimming of starlight caused by an accompanying exoplanet passing in front) and the “radial velocity” technique (the wobble of a star caused by the gravitational tugging of an orbiting exoplanet).

Although exoplanets have been discovered via microlensing before, this is the first planetary detection that uses only high cadence survey data after the event. Usually, when a microlensing event is detected, alerts are sent out to collaborating astronomers who then slew their telescopes toward the event and measurements are taken. In the case of MOA-2011-BLG-322, only data from the three surveys were used to estimate the star’s mass and the nature of the planetary companion. According to the researchers, this “shows that the survey data alone can be sufficient to characterize a planetary model.”

The exoplanet detected in this case has a mass of approximately eight times that of Jupiter and its star is likely an M-type star, around one third the mass of our sun. The exoplanet has an orbital distance of nearly 4 astronomical units (AU); or four-times the Earth-sun distance. From this observation, some interesting science can be done.

It would appear that MOA-2011-BLG-322′s massive planet exists in an orbit beyond its host star’s “snowline.” This is a region around any given star where protoplanetary material in the protoplanetary disk of a young star begins to freeze, making it a ripe environment for planets to form. However, this world appears to be too massive for its comparatively close orbit.

“According to the core accretion scenario, Jovian planets form beyond the snowlines of their parent stars, but massive planets around M-type stars should be rare, since their formation times are longer than the typical disk lifetime,” the researchers write. “In the disk instability planet-formation scenario massive planets do form around M stars, but at distances of over 7 AU.”

For such a large planet to be orbiting only 4 AU from its parent star, there appears to be some conflicts with existing planetary formation theories. Fortunately, microlensing is highly sensitive to detecting worlds beyond the snowlines of their stars, so with further discoveries by microlensing surveys will come better knowledge of how massive planets form or migrate from wider orbits.

“Unlike most of the microlensing-detected planets to date, the planet presented here was not detected in real time, but in a post-season analysis, illustrating the essence and elegance of the second-generation survey principle,” the researchers conclude.

This study has been submitted for publication in the Monthly Notices of the Royal Astronomical Society.

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